Design of Footing

Footing Design

Footings shall be of FDN foundation grade treated lumber or plywood, and gravel, crush stone or other durable materials.

The Permanent Wood Foundation System incorporates a composite footing. The footing shall be designed to distribute the live and dead axial design loads from the foundation wall to the wood footing plate to the gravel and distribute it to the supporting soil.

The size of the footing plate and the thickness and width of the granular footing shall be determined by the bearing pressure between the gravel, sand, or crushed stone and the supporting soil.

Tension perpendicular to grain stress in the footing plate is most common when the footing plate is two or more lumber sizes wider than the foundation wall bottom plate. This induced stress in the bottom face of the footing plate shall be checked. Where the tension perpendicular to grain stress exceeds the footing design, plywood reinforcement for the lumber footing plate shall be used. This can be done with strips of plywood nailed to the footing plate.

The width and thickness of the granular footing shall be, twice the width of the wood footing and the depth of gravel shall be three-quarters the width of the wood footing plate. The gravel shall be confined laterally, and extend to the undisturbed soil of the overdig hole.

All wood footing plates shall extend down to the maximum depth of frost penetration. Note: the gravel shall allways be below the frost line. The footing gravel in a basement foundation shall be drained to a sump crock by a trench filled with gravel, using a gravel filled trench to the sump crock. Footing gravel used in a crawl space or garage foundation shall be drained to the lead walls, drained down the gravel backfilled lead walls, drained through the basement floor gravel to the sump crock, only drain the gravel footing when the crawl space or garage walls are connect to a basement and the garage or crawl space footings are at a higher elevation than the sump crock in the basement. Do not drain a frost wall (sub wall) gravel footing in a walkout basement design if the sub wall gravel footing is at a lower elevation than the top of the basement sump crock gravel.

Never use a drain tile with the wood - gravel footing.

The downward axial load from the wood footing plate is assumed to be distributed outward through the gravel, sand, or crushed stone footing at an angle of 30 degrees from vertical at each edge of the footing plate.

Footing Design

Footing plate width shall be determined by the bearing pressure between the footing plate and the gravel or crushed stone footing. Minimum bearing pressure is typically 3000 psf assuming loosely compacted gravel. For 3000 psf, the footing plate width can then be determined using the following formula:

d = or > P/250= width of footing plate.

where:

• d = width of footing plate, inch.
• P = axial ASD design load, plf. (Allowable Stress Design)

Allowable axial load on footing plate per lin. ft.

2000 lbs. per sq. ft. bearing soil capacity is required.
Allowable bearing pressure on footing plate plf.

1. 2x4= 875 plf.
2. 2x6= 1375 plf.
3. 2x8= 1812 plf.
4. 2x10= 2312 plf.
5. 2x12= 2812 plf.

Point Load

When the allowable bearing pressure on the footing plate is overloaded by a point load, a load pad is designed and installed under the footing plate, this pad can be of concrete or plywood designed to tranfer the point load to the bearing soil.

Induced Tension

When the footing plate is wider than the foundation bottom wall plate, two lumber sizes or larger, induced tension perpendicular to grain stress is most common. In such case, the footing plate may be of thicker lumber or plywood strips may be used to reinforce the footing plate.

Gravel, Sand or Crushed Stone for Footing and Backfill

• Gravel shall be washed, and well graded. The maximum size stone shall not exceed 3/4". Gravel shall be free from organic, clayey, or silty soils.
• Sand shall be coarse, not smaller than 1/16" grains and shall be free from organic, clayey, or silty soils.
• Crushed stone shall have a maximum size of 1/2".

Types of Soils and Related Design Properties
(Most pwf's are designed on 2000 lbs per sq. ft. soils).

Allowable bearing value shall be check in Group 4 soils.
Presumptive load bearing values of foundation materials.
(Michigan 2000 code).
Note: Clay soils perform well with the permanent wood foundation system.

Troubleshooting Panel Wood Foundations

Foundation wall sinks out of level at any point along the foundation wall.

Causes:

a. The axial load is too heavy for the size of the footing plate.

b. There is not enough gravel depth or width under the footing plate.

c. There is too much building weight for the gravel depth under the footing plate (gravel shear). note this can happen only with undersize footing plates.

d. The footing plate is too wide giving rise too induced tension. (oversize footing plates)

e. Plywood stiffener is too thin.

f. Plywood stiffener face grain is not perpendicular to footing plate grain.

g. Plywood or concrete concentrated load pad is missing or too small for the point load.

h. The load pad is not thick enough for the concentrated load.

i. If a plywood pad, the adjacent pieces are not perpendicular to each other.

j. If a plywood pad, the adjacent pieces are not glued together.

k. The footing plate is plywood, the adjacent pieces are not perpendicular only when required, are not enough layers thick, are too wide, or are not glued together as required by some designs.

Help Desk 810 955-4305

For more information:

www.pwfs.com

www.woodfoundation.com

www.woodbasement.com

Note: Panel foundations can be site built or made in a shop. When made so it cover up items that need inspection by the Building Inspector. Third Party Inspection may be required. (inspect for grade stamps, treatment stamps, stud spacing, insulation, nailing, dip of saw cuts, plywood requirments are some of the item that a third party inspection will note.

All Panel Wood Foundations must be designed and installed in accordance with:
All current Building Codes Standards

While wood foundations are easy to build, this is only true if one is building from an accurate well designed plan. When such a plan is incorrect, or if something is left out of the plan or absent, or if a design is made using a guide manual (these are not design manuals); major mistakes can be made during the construction process. These errors cause problems for the owner, builder, and the building department.

PermanentWoodFoundation.com a service of Permanent Wood Foundation System (PWF), supported by Southern Pine lumber users affiliated with the training and installation of wood foundations.
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